Conventionally, there have been mufflers which reduce noise of fluids (for example, Patent Documents 1 and 2). In the techniques described in the Patent Documents 1 and 2, a cover is provided outside a pipe. There have also conventionally been those provided with a soundproof cover covering a muffler (a muffler sound-insulation structure) outside the muffler. According to this structure, noise of a muffler is insulated by the soundproof cover.